The Golden Wrench

A blog about bicycle repair and maintenance by the mechanics at Freewheel Bike.

Saturday, October 6, 2012

How I spent my day off

So it's Thursday evening, about two hours before close at Midtown when we receive a frantic phone call from the airport. There's a marathon this weekend, and a woman that flew into town from Spain to compete (one of Medtronic's Global Heroes) has a big problem: the airline crushed her hand cycle and broke it. After calling bike shops and wheelchair/handcycle shops all around the metro, Courage Center gave them our number and said to ask for me. The woman calling wasn't sure exactly what was broken, but would I please look at it? You bet. How much time do I have to repair the bike? It needs to be at the starting line in exactly 36 hours. Oh boy...

The hand cycle was transported from the airport to Midtown by a limousine driver a half hour later. When the driver opened the trunk, I had to ask him if this was the whole bike, or if perhaps we're missing the rest of it. I'd never seen such an alien unicycle-like contraption before. This was the whole thing, he assured me. He said he's done some unusual runs before, but this one probably tops the list. Once I got it down to the bike shop and examined it more closely, I deduced that it clamps onto the front of a wheelchair to turn it into a tricycle. I've worked on several hand cycles, but never this type before.

My heart sank a little when I examined the broken part; the non-drive crank arm was snapped in two. And this is no normal crank arm; it's made from the sawed-off hub portion of a normal square-taper crank with a custom arm welded on. It broke at the weld, an inevitable weak point.

If it was made of steel, I could have it over to Peacock Groove
and Erik Noren would have it TIG welded back together in no time flat.
But it's aluminum, and nobody I know is willing to do weld repairs
on aluminum bike parts due to the tricky metallurgical complexities involved. The manufacturer, Stricker Handbikes,
is straight out of Germany, and their website is only partially
translated to English. I doubt they'll be able to help me out in the
tight timeframe available. Thus far, no spare parts or loaner bikes could be located
anywhere nearby, so I began formulating a plan. This was no normal bike shop repair; I would be spending my day off at TC Makers' Hack Factory, a shared shop space in South Minneapolis. They have the precision heavy machinery, and I have my personal collection of tooling and accessories. Time to make a part from scratch...

At about 11am Friday, I paid a visit to Amble's Machinery & Hardware and picked up a block of aluminum billet the size of a kleenex box. It was the smallest piece Jim had that would do the job, but it was still much larger than I needed, so I sliced off a piece in the bandsaw to work with. My hand is for scale; I don't normally put my hand in the bandsaw...

This is an old Shimano non-drive crank from the junk bin. It's going to donate a square-taper interface to the cause so I don't have to try to machine the taper and extractor threading, which I'm not equipped to handle. I faced the back of the hub, bolted it to a 1-2-3 block, and clamped the block in the Bridgeport vice at an angle so the original crank surface was level. Then I machined the top perfectly flat and created two parallel flat surfaces on the sides.

Here's that piece of billet again. I faced all six sides flat and square, then started making a groove down the middle the exact width of the squared-up section of crank arm. Careful measurements and a few very light finishing passes later, I had a perfect slide-on fit with less than a thousandth of an inch of play.

After some careful angle measurements on the unbroken drive-side crank, I decided where to cut off the excess and set up the bandsaw at a 30 degree angle. I left plenty of extra material for later when I would be adding pinch bolts. Just ask any barber; you can always take a little more off later...

Back at the mill, I used a 30 degree angle block to machine a flat surface on the block and step-drilled a hole to within 1/8" of the diameter of the broken crank arm shaft. I neglected to take a photo while finishing the hole to final diameter with an adjustable boring head, measuring progress with a telescopic bore gauge and my favorite digital micrometer. I stopped when I was within a thousandth of an inch to check fit with the actual part. Too small; take another thousandth off. A perfect snug sliding fit!

Everything fell into place in the nick of time for this repair. I was
interrupted from machining about a dozen times during the day by a representative of
the airline requesting info about the bike to try and find parts or a
loaner. When I was about 2/3 finished with the part, the representative finally called for
the last time to inform me her leads were all spent, and she was giving
up. Now this strange adapter I'm making from plans in my head is the only option. The day is passing quickly; it's crunch time. I stopped documenting my progress and worked as quickly as I safely could.

Here's the mostly-complete part, sitting on the remainder of the billet from which it came. I have already drilled holes for pinch bolts to go through the stub of the new crank arm and to clamp down on the cylindrical old crank arm part. I would have made it lighter by taking off more material here and there, but I was running out of time.

Here's the finished assembly at about 7pm Friday, after a whole day in the machine shop. I did the concept and design work in my head, made a few simple sketches, and pretty much went from one logical step to the next until I had a working part. If I goofed up anywhere along the way, I certainly wouldn't have time to start over again. Fortunately, everything went perfectly.

This is Anna and I at about 8pm Friday night, in the lobby of the Crowne Plaza Hotel in downtown St. Paul. I hand-delivered the repaired Stricker handbike and did the final assembly in the lobby. Anna was very excited to be back in action after fearing that she was out of luck. After a few quick shifting adjustments and a crude on-the-spot derailleur hanger alignment with pliers, she was ready to tackle the race course in the morning.

I have to stress that this particular project is not the kind of repair we typically do at
Freewheel, but we do have excellent resources and connections for when
push comes to shove and the clock is ticking. If you need hand cycle repair work (that hopefully doesn't involve fabricating parts!) give me a call at Freewheel's Midtown shop and we'll talk options. 612-238-4447; ask for Karl.

Anna, I hope you enjoy the rest of your trip, and let me know how the bike performs!